Multiple sulfur isotope studies of the oldest rocks in South Korea: Possibility of recycling of ancient sulfur in Daeijak Island.

Abstract:

Mass-independent fractionation processes of sulfur isotopes are reported from the rocks older than 2.0 Ga in Earth history. In those old rocks, large Δ³³S anomalies and the variations are reported: before 2.45 Ga, the variations of values are greater than ±1‰; between 2.45 Ga and 2.0 Ga, the variations have a range from -0.1 to 0.5‰; after 2.0 Ga the variations have more smaller range from -0.1 to 0.2‰. Sulfur isotope values from the rocks before 2.0 Ga may show anomalous Δ³³S values from the ancient sulfur reservoir.

In South Korea, the oldest rocks (ca. 2.5 Ga) are found in Daeijak Island. Major lithology of island consists of gabbroic amphibolite, tonalitic and granodioritic gneiss, and leucocratic granite, which belongs to the Gyeonggi massif. The oldest age was measured in the core of the zircon from the tonalitic protolith. Some sulfide minerals are found in amphibolite and small amount of sulfur is also included in other rocks.

As a preliminary study of Δ³³S analysis in sulfides from the oldest rock in Korea, analysis of samples from Daeijak Island was conducted. The samples used for the analysis were amphibolite, gneiss, and leucocratic granite. Sulfur components were extracted from the each whole rock samples using CRS and thode solution. Then it was converted to SF₆ to measure sulfur isotope ratios at the mass spectrometer. Δ³³S values of samples are measured to give the range from -0.009 to 0.014‰. Multiple sulfur isotopes were examined for the tracers of sulfur source and sulfur cycles in ancient times.